1. Field of Invention
The present invention relates to a process for marking the surface of a rolling mill roll in order to provide the said roll with an isotropic and durable textured surface. The invention also extends to a roll having a surface marked by means of this process and to a sheet metal rolled with such a roll.
As is known, the texture of a roll determines, to a large extent, the texture of sheet metal rolled with the said roll.
The texture of the roll is formed by a multitude of microscopic grooves and ridges which respectively imprint elevations and grooves in the surface of the sheet metal. The regularity of the dimensions and the distribution of this texture affects, in particular, the suitability of these sheet metals for stamping and coating.
2. Description of the Prior Art
From the Pat. No. BE-A-870 600 in particular, a process is already known for creating a multitude of microscopic dents or microcraters in the surface of a rolling mill roll by means of an intermittent laser beam. These microscopic dents are surrounded by a projecting rim or ridge having a high hardness value. A sheet metal rolled with such a roll therefore has, on its surface, a multitude of grooves imprinted by the ridges of the roll and elevations corresponding to the microscopic dents.
In this process, the roll is set in rotation about its longitudinal axis and its surface is struck by a series of laser impulses produced by chopping a continuous laser beam by means of a pierced disk. The laser beam moves along the roll in such a manner that its impact zone describes a helicoidal trajectory on the roll surface.
The device for forming such a microscopic dent is now well-known. It should be recalled that in its impact zone on the roll surface, each laser impulse causes a certain volume of metal to be heated and, within this volume, a droplet of the metal of the roll is melted. The volume of heated metal, as well as the volume of molten metal, depend on the strength and duration of the laser impulse. The action of the laser impulse is generally reinforced by a jet of oxidising gas directed at the said impact zone. The sudden local increase in temperature causes the formation of a plasma above the impact zone due to combustion of the metal. This plasma exerts a pressure on the molten metal which, combined with the mechanical effect of the gas jet, pushes back the molten metal radially towards the edge of the heated zone. The molten metal thus pushed back, in some cases even beyond the heated zone, forms a ridge which solidifies very quickly. The cavity hollowed out by the pushing back of the molten metal forms the microscopic dents.
The ridges and microscopic dents collectively determine the texture of the roll surface and hence the texture of the sheet metal to be rolled with the roll. The service life of the ridges therefore affects not only the service life of the roll and the evenness of texture of the sheet metals produced, but also, as a result thereof, the evenness of the properties of these sheet metals, in particular with respect to their suitability for painting and resistance to jamming.
The service life of the ridges depends, to a large extent, on their adhesion to the roll surface. This adhesion is in turn the result of a welding operation: if the droplet of molten metal is pushed back too far beyond the area heated by the laser impulse, the ridge formed will not adhere correctly to the roll surface, the temperature of which is locally too low. This will cause a crack at the interface between the roll surface and the ridge, leading to a rapid pulling-off of the ridge due to the force of the rolling process. The quality of the laminated sheet metals then deteriorates and it is necessary to interrupt the rolling process to redress the roll. The productivity of the rolling mill may thereby be seriously affected.
Furthermore, the process according to the cited No. BE-A-870 609 ensures the formation of joining microscopic dents, such that their ridges form, in the roll surface, a network of ridges surrounding a multitude of individual depressions. Consequently, a sheet metal rolled with such a roll will have, on its surface, a network of communicating grooves surrounding individual, regularly-distributed elevations.
This network of grooves generally assures a good circulation, and hence an homogeneous distribution, of the lubricant used, for example, for a stamping operation of the sheet metal. It has been shown, however, that if the pressure of the equipment becomes too strong, the lubricant is forced back into the network of grooves and forms direct contacts between the metal of the equipment and the metal of the surface elevations of the sheet metal. The latter are then exposed to abrasion and pulling-off, involving a risk of degradation of the surface of the sheet metal. Furthermore, the lack of lubricant increases the risk of jamming in the case of stamping.